Arterial pulse measurement using electret condenser microphone requires the standard to validate the value of the measurement. This standard requires the test device to reproducing the mechanical vibration to emulate the arterial pulse vibration. The main objective of this paper is to discuss the test device of electret condenser microphone using class A amplifiers and low-frequency loudspeaker. To validate this pulse measurement, this class A amplifier is examined under an experimental setup. The experiments showed that the device can be used as an alternative solution to generate the mechanical signal source to simulate the human arterial pulse.

1. TELKOMNIKA Telecommunication, Computing, Electronics and Control
Vol. 18, No. 3, June 2020, pp. 1368~1375
ISSN: 1693-6930, accredited First Grade by Kemenristekdikti, Decree No: 21/E/KPT/2018
DOI: 10.12928/TELKOMNIKA.v18i3.14831  1368
Journal homepage: http://journal.uad.ac.id/index.php/TELKOMNIKA
Low-frequency response test device
of electret condenser microphone
Erni Yudaningtyas1
, Achsanul Khabib2
, Waru Djuriatno3
, Zakiyah Amalia4
,
Ramadhani Kurniawan Subroto5
1,3,5
Department of Electrical Engineering, University of Brawijaya, Indonesia
2,4
Department of Electrical Engineering, State Polytechnic of Malang, Indonesia
Article Info ABSTRACT
Article history:
Received Aug 2, 2019
Revised Jan 31, 2020
Accepted Feb 26, 2020
Arterial pulse measurement using electret condenser microphone requires
the standard to validate the value of the measurement. This standard
requires the test device to reproducing the mechanical vibration to emulate
the arterial pulse vibration. The main objective of this paper is to discuss
the test device of electret condenser microphone using class A amplifiers and
low-frequency loudspeaker. To validate this pulse measurement, this class A
amplifier is examined under an experimental setup. The experiments showed
that the device can be used as an alternative solution to generate the mechanical
signal source to simulate the human arterial pulse.
Keywords:
Class A amplifier
Electret condenser microphone
Loudspeaker
Low-frequency
Test device This is an open access article under the CC BY-SA license.
Corresponding Author:
Erni Yudaningtyas,
Department of Electrical Engineering,
University of Brawijaya, Indonesia.
Email: erni@ub.ac.id
1. INTRODUCTION
An electret condenser microphone (ECM) is a microphone designed in the audio frequency range
about 20 Hz-20 kHz [1, 2]. ECM is utilized to accurately measure the arterial pulse in the low-frequency i.e,
1 Hz [2, 3]. The arterial pulse frequency is around 1.5-2.1 Hz at the normal condition of human [4-8].
The ECM works to detect the arterial pulse and is operated by taking a mechanical signal from the arterial
pulse [3]. This ECM is used to measure arterial pulses frequently [9-13]. The arterial pulse measurement is
based on the Traditional Chinese Medicine method [14-16]. Even though ECM has been conducted by many
kinds of literature, the research on ECM is still relevant today. In [3], the arterial pulse recording device has
been designed from ECM using a mechanical filter and electronic filter [17]. In practical applications, this
ECM requires a tool to determine the response of electrical and mechanical responses such that it can be
known whether this ECM is stationery at the frequency of 0.5-10 Hz. By considering this problem, the test
device is required to examine and obtain the frequency response of ECM or even can be used as a human
pulse emulator.
Since the pulse frequency is typically low frequency, between 0.5-2 Hz, special treatment is
implemented to satisfy these frequency ranges. The previous works related to ECM for the application
of low-frequency device has not yet been conducted. The test device based ECM to obtain the frequency
response is constructed in this paper. The test device generates the mechanical signal coming from
the loudspeaker. In this paper, the type of loudspeakers used is an oval woofer type from the TV [18].

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The loudspeaker is then operated in the frequency range 0,5-10 Hz for mechanical vibration such that it can
be used as a pulse emulator. The additional device that is able to drive the loudspeaker is often called a signal
amplifier or an amplifier.
The class A amplifier is selected to eliminate the distortion of the signal. Class A amplifier has
continuous current flows without any distortion because the transistor is operated continuously [19]. This
type of amplifier is a kind of amplifier exhibiting low efficiency as the current flowing in the transistor is
continuous. This study discusses the displacement response on a test device when given a low-frequency
signal from a signal generator amplified by the class A amplifier. Displacement amplifier used as
a mechanical signal source is also utilized as a pulse emulator and mechanical signal generator. Figure 1
shows the configuration of the low-frequency response test the device of ECM using a low-frequency
amplifier and loudspeaker. In front of the loudspeaker, soft silicon rubber is installed and responsible to
increase mechanical coupling between the loudspeaker and ECM. The detail description of the mechanical
coupling has been discussed in [3].
Figure 1. The configuration of the low-frequency response test device of ECM
based low-frequency loudspeaker [3]
To observe the speaker impedance, the impedance test is required to measure the speaker impedance
in the range frequency of 0,5-10 Hz. In this study, the impedance of the speaker is tested using a vector
network analyzer (VNA) [20, 21] type Bode-100. In addition, the low-frequency amplifier is analyzed to
observe the impedance of the amplifier. This paper discusses test devices using low-frequency amplifiers
and low-frequency loudspeakers to measure the response of ECM for arterial pulse sensor [22, 23].
The experimental result has verified the proposed benchmarks.
This paper organized as follows: section 1 introduces the background of this research which is about
the introduction of arterial pulse sensors using ECM and how the background of the test device is made to
determine the frequency response of the ECM that is used as an arterial pulse sensor. Section 2 introduces the
material and the method used in designing arterial pulse test devices, the material consists of low-frequency
loudspeaker and low-frequency amplifiers. The method describes how to operate the arterial pulse test device.
The Results and Discussion are discussed in section 3. And the last, section 4 presents the Conclusion.
2. MATERIAL AND METHOD
To implement the proposed test device, some components are realized to this test device such as
the loudspeaker with low-frequency characters, the amplifier without distortion and has high linearity
and measurement device i.e. oscilloscope and vector network analyzer. The detailed components of
the overall system are explained as follows.
2.1. Low-frequency loudspeaker
Commonly, the existing speaker has a frequency response in the range between 20 Hz to 20 kHz,
however, in this study, the proposed speaker is designed to satisfy the frequency range of 0.5-10 Hz. Figure 2
shows the schematic equivalent circuit of the proposed speaker. The proposed speaker consists of two
components, i.e: the electrical and the mechanical component. The electrical component is composed by
the moving coil, while the mechanical component consists of diaphragm and spring.
According to Figure 2, the mathematical model of the loudspeaker can be derived as follows:
𝑆𝑝( 𝜔) = 𝑗𝜔𝐿 + 𝑅𝑒 +
1
𝑗(𝜔𝐶𝑚−
1
𝜔𝐿𝑚
)+
1
𝑅𝑚
(1)
where the loudspeaker impedance is represented by 𝑆𝑝(𝜔), the resistance of the loudspeaker coil is assumed
by 𝑅𝑒, the inductance of the loudspeaker coil is denoted by 𝐿𝑒, the mass of diaphragm is represented by 𝐶𝑚,

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the spring diaphragm of the loudspeaker is related by 𝐿𝑚 and the losses of spring diaphragm is represented
by 𝑅𝑚. The parameters of the low-frequency loudspeaker are described in Table 1.
Figure 2. The Modeling of Loudspeaker
Table 1. The parameter of
the loudspeaker modelling
Parameter Description Value
𝑅𝑒 Loudspeaker coil resistance 6.1 Ω
𝐿𝑒 Loudspeaker coil inductance 0.11 mH
𝐿𝑚 Spring diaphragm of the
loudspeaker
7.5 mH
𝑅𝑚 Spring diaphragm losses 8.9 Ω
𝐶𝑚 Diaphragm mass 306 µF
Amplifier Class A Amplifier 1-1000
Hz
According to Figure 2, the loudspeaker has a resonant frequency. The resonant frequency is
the relationship between the mechanical resonant frequency of the diaphragm and the moving coil
represented in (2) as follows:
𝑓𝑟𝑒𝑠 =
1
2𝜋
√
𝑆
𝑀
(2)
where 𝑓𝑟𝑒𝑠 is the resonant frequency of the loudspeaker, 𝑆 is the stiffness of the spring compliance
loudspeaker and 𝑀 is the mass (weight) of all components moving on the loudspeaker. 𝑆 is a notation for
the loudspeaker stiffness system in centimetres per dyne which consists of a spider or spring system that
positions the moving part again at the rest position. This moving part position can move forward
or backwards from the rest position according to the signal given to the loudspeaker. The strength of this
spring will determine how much force is given by voice-coil. The greater the value of 𝑆, the greater the force
that the moving coil must be given. Whereas, 𝑀 represents all components loaded from loudspeakers such as
diaphragms, cones, and moving coils as well as included in this 𝑀 component. Based on the aforementioned
explanation, it can be concluded that the type of woofer loudspeaker has the character of S which is lower
than other types of loudspeakers and 𝑀 is greater than other types of loudspeakers.
To find out the force acting on the diaphragm of the loudspeaker provided by the voice coil,
the combination of the electrical and mechanical domain must be incorporated and added. Magnetizing force
to obtain a large force that affects the moving force of the loudspeaker represented in the following:
𝑓 = 𝐵𝑙𝑖
𝑒 = 𝐵𝑙𝑢 (3)
where 𝐵 is the magnetic field strength (𝑇), 𝐼 is the length of the conductor in the magnetic field (𝑚),
𝑓 is the force in Newton (𝑁), 𝑢 is the speed from moving the moving part loudspeaker (𝑚/𝑠), 𝑖 is the current
passing through the moving coil from the loudspeaker (𝐴) and 𝑒 is the voltage supplying moving coil from
the loudspeaker (𝑉). Mechanical quality factor and electrical quality factor are represented this following (4):
𝑄 𝐸𝑆 = 2𝜋𝑓𝑆 𝐶 𝑚 𝑅 𝑒
𝑄 𝑀𝑆 = 𝑓𝑆 𝐶 𝑚 𝑅 𝑚 (4)
𝑓𝑆 =
1
2𝜋√ 𝐶 𝑚 𝑅 𝑚
where 𝑄 𝑀𝑆 represents the mechanical quality factor, 𝑄 𝐸𝑆 denotes the electrical quality factor and 𝑓𝑆 is
the resonance frequency.
2.2. Low-frequency amplifier
Since the audio amplifier [24] has a frequency response in the range frequency of 20-20 kHz,
the amplifier is conditioned in the range frequency of 0.5-10 Hz. In this paper, the basic principle of
the low-frequency amplifier using modified Class A is introduced. The schematics of the circuit are
shown in Figure 3 and Figure 4. The proposed amplifier is composed by Darlington transistors and several
passive components.

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Figure 3. The proposed of low-frequency amplifier
based class-A amplifier
Figure 4. Signal conditioner
The parameters of the proposed amplifier are listed in Table 2. 𝑅 𝐵 is a resistor that acts as a buffer
between the signal source and the main circuit such that it does not overload the signal generator, 𝑅 𝑉𝐵
and 𝑅 𝐵𝐺 is a voltage divider that provides voltage and current to the base of 𝑇1 transistor, 𝑅 𝐶 is a resistor that
regulates the current of Darlington transistors 𝑇1 and 𝑇2. RE is the resistor to compensate the deviation
of Darlington transistors 𝑇1 and 𝑇2, CE is to compensate for the sine wave signal and the 𝑅 𝐵2 is the resistor to
compensate between the loudspeaker and amplifier.
Table 2. The parameter of proposed amplifier
Parameters Value
𝑅 𝐵1 100 Ω
𝑅 𝑉𝐵 10 KΩ
𝑅 𝐵𝐺 1 KΩ
𝑅𝑐 8 Ω
𝑅 𝐸 24 Ω
𝑅 𝐵2 14 Ω
𝑇1 2N3035
𝑇2 2N3055
𝐶 𝐸 3.3 µF
To determine the voltage of amplifier affecting the force of loudspeaker, the force of motor using
Ohm's law relating to a Lorentz actuator can be calculated. The motor voltage must be replaced by a short
circuit to determine the impedance of a voltage source [25]. The force of loudspeaker which affecting
the amplifier voltage can be described by
𝐹 = 𝐵𝑖𝑙 =
𝐵𝑉𝑠 𝑙
𝑅 𝑐
[𝑁] (5)
where 𝐵 is the flux density of the loudspeaker magnetic field around the coil in Tesla [𝑇]. 𝑙 is the length
of the voice coil inside the magnetic field. To determine the dynamic response 𝑇𝐹,𝑥 of the cone displacement
𝑥 which related to mass 𝑚 to drive the force of excitation 𝐹 can be described by
𝑇𝐹,𝑥( 𝜔) =
𝑥
𝐹
=
𝐶𝑠
−
𝜔2
𝜔0
2+2𝑗𝜁
𝜔
𝜔0
+1
(6)
where 𝜔 = 2𝜋𝑓, ζ is damping ratio, 𝐶𝑠 is compliance and the resonant frequency of mechanical part 𝜔0 is
calculated by: 𝜁 =
𝑐
2√𝑘𝑚
, 𝐶𝑠 =
1
𝑘
and𝜔0 = √
𝑘
𝑚
.
The resonant frequency of the mechanical part is called "eigenfrequency" [11] and denotes as 𝑓0.
The 𝑓0 is equal to 𝜔0/2𝜋 called by fundamental resonant frequency. Around its the fundamental frequency,
the loudspeaker actuator is matched with the force due to the displacement of the cone against the stiffness
of the loudspeaker suspension. In this study, the loudspeaker resonant frequency 𝑓0 is 193 Hz, but
the loudspeaker is operated at a lower frequency in the range of 0.5-10 Hz by designing a special amplifier
that is able to operate at a very low frequency such that can drive the motor force of loudspeaker.
The complete description of the experimental setup of the low-frequency amplifier is later discussed in
the following section.

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2.3. Experimental setup
This study proposes a new test device for testing a performance of the ECM that operates in
the frequency of arterial pulse region. In addition, test devices can be used for mechanical pulse emulator
the same as the arterial pulse. To design the test device, the researchers conduct an experiment by measuring
the impedance and gain of loudspeaker using a Bode-100 type vector network analyzer (VNA). Based on
the experiment, it can be obviously seen that the loudspeaker impedance rate in the frequency
range 0.5-10 Hz. In addition, the gain of the loudspeaker is also measured using this VNA in the frequency
range 0.5-10 Hz.
To characterize the Class A amplifier, the output voltage on the amplifier is conditioned constant
even though the input frequency is varied in the range of 0.5-10 Hz. This output voltage is then represented
by displacement of the diaphragm of the loudspeaker. This displacement is then conditioned at a desired
constant value by varying the input voltage of the signal generator. In order to verify the performance
of the proposed test device, a novel class A power amplifier design and a low-frequency loudspeaker are
verified in Figure 5. The test benchmark consists of several components i.e: signal generator, digital storage
oscilloscope, power supply, Class A amplifier and loudspeaker. Each of those components has its function.
The signal generator produces the sine wave. The Class A amplifier is supplied by the power supply,
the loudspeaker generates the mechanical vibration driven from the Class A amplifier. To measure
the amplitude response, digital storage oscilloscope (DSO) is connected to the terminal 𝑉𝑖𝑛 and 𝑉𝑜𝑢𝑡 of
Class A amplifier. The experimental setup parameters are listed in Table 3.
The experimental setup of the proposed system is clearly shown in Figure 5, while the detailed test
device is shown in Figure 6. The ECM stand is the ECM holder aiming to support and maintain the ECM
position. ECM is the focus of research which is the main objective in this study. Soft silicon rubber is utilized
to adjust mechanical coupling between the loudspeaker and ECM diaphragms. The description of the ECM
system as an arterial pulse sensor is explained in detail in [3]. In this research, the main topic is the design
of a test device to generate a mechanical signal such as human pulse arterial signals which the loudspeaker is
conditioned in the frequency range 0.5-10 Hz with a displacement of 0-1.5 𝑚𝑚 𝑝𝑝.
Table 3. The parameter of experimental setup
Parameters Specification
Power Supply 20 V DC
Function generator KMOON, Dual-channel DDS signal Generator
DSO Hantek MSO5074F, 4 channel DSO, 70 MHz
Figure 5. The photograph of the low-frequency
response test device of the ECM
Figure 6. The configuration of loudspeaker
and ECM
3. RESULT AND DISCUSSION
Figure 7 depicts the loudspeaker impedance measurement using VNA type Bode 100 from Omicron
Lab. In this impedance measurement, the loudspeaker is stationary and there is no object hitching on this
loudspeaker. Then, the VNA is set to kick the loudspeaker in the range 0.5-10 Hz. The frequency response
of the loudspeaker is measured by VNA and shown in Figure 8. Based on the measurement, the loudspeaker
has the impedance at about 7.09 Ω. Figure 8 shows the loudspeaker gain in the frequency range of 1-10 Hz.
However, the loudspeaker gain tends to be constant at about 0.2477 in the frequency range of 3-10 Hz.
In future, it can be interpreted in the frequency range of 310 Hz that this loudspeaker has a gain of 0.2477 dB.

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Figure 7. The loudspeaker impedance measurement using VNA
Figure 8. The loudspeaker gain measurement using VNA
The voltage ratio between the voltage signal generator which represented as 𝑉𝑖𝑛 and the amplitude
voltage at loudspeaker which denoted as 𝑉𝑜𝑢𝑡 in every 0.5 Hz sampling is shown in Figure 9. It can be clearly
seen that the ratio of the test device system tends to decrease when the frequency is getting higher. It implies
that the test device system will require larger 𝑉𝑖𝑛 if the frequency is raised. Figure 9 shows that the frequency
response value is not fixed. Therefore, signal conditioners as shown in Figure 4 are added as shown in Figure
10 indicating the input voltage 𝑉𝑖𝑛 from a signal generator in every 0.5 Hz. Figure 11 shows the frequency
response after signal conditioners from Figure 10 are added. Consequently, the diaphragm displacement of the
test device is stationary at the range 3.36 to 3.46 at the frequency range of 0.5-10 Hz as depicted by Figure 11. It
can be clearly seen that the loudspeaker is operated at low frequencies and can be used as a test device.
Figure 9. The voltage ratio between the voltage signal generator which represented as 𝑉𝑖𝑛 and the amplitude
voltage at speaker which denoted as 𝑉𝑜𝑢𝑡 in every 0.5 Hz

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Figure 10. The voltage of the signal generator which denotes as V_in in every 0.5
Figure 11. The diaphragm displacement of the loudspeaker in 𝑚𝑚 𝑝𝑝
4. CONCLUSION
The low-frequency response test devices of ECM has been successfully implemented. The test
device has been successfully verified with constant displacement. So the proposed test device is able to used
as a low-frequency response test device.
ACKNOWLEDGEMENT
The research project was supported by the Department of Electrical Engineering University
of Brawijaya.
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